Was this ever resolved? I wonder if you could paste the code you are using for that page in a comment below. I created a couple of sandbox pages looking to replicate your error and couldn't seem to work it out. I tried two approaches, embedded Canvas Studio videos and just uploading mp4 files directly. Both seemed to work.
Regarding the width and height, that can be done with both CSS and HTML. I prefer CSS, which may look like:
<p><iframe style="width: 400px; height: 225px;" src="URL"></iframe></p>
HTML would be more like
<p><iframe width="400px" height="225px" src="URL"></iframe></p>
Sometimes there is both HTML and CSS, so you'd want to remove one and avoid conflict.
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